Method of preparing a thiophene-containing or furan-containing conjugated compound and precursor compound used therein

ABSTRACT

The invention relates to a method of preparing thiophene-containing or furan-containing conjugated compounds such as polythiophene. The method uses a precursor compound having tetrahydrothiophene or tetrahydrofuran precursor units having arylthio or alkylthio substituents. The precursor units can be thermally converted into thiophene or furan units. Due to the presence of the precursor units the precursor compound is soluble and can, unlike the corresponding conjugated compound, be processed from solution.

FIELD OF THE INVENTION

The invention relates to a method of preparing a thiophene-containing orfuran-containing conjugated compound, whereby a soluble precursorcompound, comprising a precursor unit having thermally separablesubstituents, is heated such that said thermally separable substituentsare eliminated from said precursor compound while converting saidprecursor compound into said thiophene-containing or furan-containingconjugated compound. The invention also relates to precursor compoundssuitable for use in such a method. The invention further relates to asemiconductor device in which use is made of a conjugated compoundobtainable by using such a method.

BACKGROUND OF THE INVENTION

Conjugated compounds, or more in particular, thiophene-containing orfuran-containing conjugated compounds are used in various industrialapplications. For example, they can be used as dyes or pigments, as(semi)conductors, (electro)luminescent material or in electr(on)ical,optical and electro-optical devices such as light emitting diodes,field-effect transistors, solar cells, polarizing optical elements andbatteries. In the context of the invention a compound is considered tobe conjugated if, upon electronic excitation, it absorbs ultravioletlight or radiation of lower frequencies.

However, due to the inherent rigidity of the conjugated system, manypotentially interesting conjugated compounds are insoluble. For manyindustrial applications processability from solution and thereforesolubility is an essential requirement if an economically viable processis to be obtained.

To enhance the solubility of a conjugated compound it has been proposedto include solubilizing substituents such as large alkyl or alkoxygroups. However, this has the undesired side-effect that propertiesother than the solubility, such as the charge-carrier mobility, areadversely affected as well.

In order to enhance the processability from solution, it has also beenproposed to use a method of the type mentioned in the opening paragraph.In such a method, referred to as a precursor method for short, theprocessing, such as, for example, the formation of a layer byspincoating, is done using the precursor compound. As a final step, theprecursor compound is (thermally) converted by separating the separablesubstituents from the remainder of the compound thus forming theconjugated compound. Only a few conjugated compounds have been preparedby a method which uses such precursor compounds. One example ispoly(2,5-thienylenevinylene) as disclosed in a publication by Kwan-YueJen et al. in J. Chem. Soc., Chem. Comm., 1987, p309. Since theprecursor method is a very attractive method of rendering conjugatedcompounds processable from solution, there still exists a need formethods of preparing a conjugated compound using precursor compounds.

SUMMARY OF THE INVENTION

It is an object of the invention to provide, inter alia, a method ofpreparing a thiophene-containing or furan-containing conjugated compoundfrom a precursor compound which is processable from solution andthermally convertible to said thiophene-containing or furan-containingconjugated compound.

The object of the invention is achieved by a method of the typementioned in the opening paragraph which, according to the invention, ischaracterized in that the precursor unit used is a tetrahydrothiopheneor tetrahydrofuran unit having thermally separable substituents --SR₁and --SR₂, wherein R₁ and R₂ are independently selected as an alkyl oraryl group. In the method according to the invention use is made ofthermally convertible precursor compounds having precursor units which,after elimination of the thermally separable substituents by heating toan adequate temperature of, typically 200° C., are converted intothiophene or furan heterocycles. The presence of the precursor unitsenhances the solubility of the precursor compound of which they are partrelative to the conjugated compound which contains the correspondingthiophene or furan units. The precursor units can be suitably used inany method of preparing a thiophene or furan-containing compound butpreferably they are used to prepare conjugated compounds. Precursorunits which can be converted into thiophene or furan units were hithertounavailable and allow conjugated compounds which are known to be ratherintractable, such as unsubstituted polythiophene, to be processed asthough they are soluble.

The improved processability is most advantageously exploited inconjunction with a substrate. A preferred embodiment of the methodaccording the invention is therefore characterized in that, before theprecursor compound is heated, a solution comprising the precursorcompound is prepared and provided onto a substrate. Suitably, thesubstrate is made, for example, of glass, quartz, silicon or a syntheticresin. The conjugated compound can be processed in the form of a layerby any conventional method which uses a solvent, for example, byspincoating a solution of the precursor compound onto the substrate.

Heating is suitably performed by conventional means such as an oven or ahot plate. Though not essential, heating may be performed under reducedpressure or under an inert atmosphere.

The precursor compound used in the method according to the inventioncontains precursor units derived from the heterocyclestetrahydrothiophene or tetrahydrofuran. Since in mostthiophene-containing or furan-containing compounds the thiophene orfuran ring is substituted at both the 2 and 5 position, use is made inparticular of precursor units which are substituted correspondingly bynon-separable substituents.

In addition to said substituents, the precursor unit has thermallyseparable substituents. By heating or, depending on the type ofseparable substituent, any other suitable method such as irradiation,these substituents are separated from the precursor unit. The separationtakes place by way of a β elimination reaction which involves two groupson adjacent atoms, one group of which is the thermally separable group.The reaction produces a double bond on the heterocycle, thus convertingthe tetrahydrothiophene or tetrahydrofuran into the correspondingdihydro heterocycle. Two thermally separable substituents suffice toconvert a tetrahydrothiophene (tetrahydrofuran) into its thiophene(furan) analog. The first of said two thermally separable substituentscan be located at the 2 position, but from a synthetic point of view the3 position is preferred. Analogously, the second thermally separablesubstituent is located at the 4 position.

Examples of thermally separable substituents are sulfone, sulfoxide,alkoxy, aryloxy, such as phenoxy, amino, --NR₃ ⁺, or --SR₂ ⁺ groups.Suitable precursor units are tetrahydrothiophene or tetrahydrofuranunits having thermally separable substituents --SR₁ and --SR₂, whereinR₁ and R₂ are independently selected as an alkyl or aryl group.

Preferably, a precursor unit is used according to the formula (I)##STR1## wherein X is equal to O or S, and R₁ and R₂ independentlyselected as an alkyl or aryl group. By heating to a temperature of,typically, 200° C. or higher, the thermally separable groups --SR₁ and--SR₂ are eliminated thus forming the sulfides R₅ SR₁ and R₆ SR₂ and aconjugated unit in the form of a thiophene (X═S) or furan (X═O) unithaving substituents R₃ and R₄ located at the positions 3 and 4respectively. The substituents R₃ and R₄ can be varied so as to obtain arange of conjugated compounds which can be rendered processable fromsolution. Since the tetrahydro unit itself is not conjugated, which hasa solubility enhancing effect, and the substituents R₁, R₂, R₅, and R6enhance the solubility as well, the choice of substituents R₃ and R₄ isnot limited by solubility considerations. Suitable choices are forexample methyl, ethyl, (m)ethoxy, nitro or hydrogen.

The substituents R₁, R₂, R₅, and R₆ are not part of the conjugatedcompound which is obtained after thermal conversion and can, forexample, be used to influence the temperature at which the thermalconversion occurs or can be chosen so as to obtain a convenientprocessing or synthetic route. Preferably, they are chosen such that thesulfide to be eliminated vaporizes at the temperature at which thethermal conversion is carried out.

A suitable choice of R₅ and R₆ is hydrogen. Substituents R₁ and R₂ arepreferably selected from the group consisting of phenyl, 4-methylphenyl,4-chlorophenyl, 4-nitrophenyl, 4-pyridyl, ethyl and tert-butyl.Surprisingly, the rate and temperature of conversion is almost the samethroughout this group. Also, the thermal conversion proceeds faster ifthe substituents --SR₁ and R₅ (or --SR₂ and R₆) are in a transconformation with respect to each other.

The thermally separable substituent can also be eliminated from theprecursor compound by using a base as a catalyst, preferably insolution.

In order to reduce the number of isomers of the precursor compoundswhich will be formed during synthesis, R₁ is preferably chosen equal toR₂. Reducing the number of isomers makes the synthetic procedure and inparticular the purification of crude products much simpler.

A preferred method according to the invention is characterized in thatuse is made of a 2,5-dithienyltetrahydrothiophene or2,5-dithienyltetrahydrofuran unit as the precursor unit. Said precursorunits are convenient building blocks in the synthesis of precursorcompounds of which use is made in methods according to the invention.For example, the thiophene rings can be selectively brominated at the(unsubstituted) 2 and 5 positions. The (di)bromo compounds thus obtainedcan be suitably used in a large variety of well known couplingreactions, such as the palladium catalyzed reaction between a bromocompound and an aromatic stannane (see for instance, J. K. Stille,Angew. Chem., Int. Ed. Engl. 1986, 25, p508), or the coupling of twobromo compounds using a nickel-based catalyst (see for instance, K.Chmil et al in Makromol. Chem. Rapid Commun. 1993, 14, p271)

Another preferred embodiment of the method according to the invention ischaracterized in that the thiophene-containing or furan-containingconjugated compound being prepared is a polymer having a substantiallyconjugated backbone. In the context of the invention, the term polymerincludes oligomer. Processability from solution is a property which manya polymer having a substantially conjugated backbone does not possess.Due to the presence of an extensive conjugated system, in many cases apotentially interesting conjugated polymer compound cannot besynthesized at all or only if it has a low molar mass. The methodaccording to the invention can therefore be applied with particularadvantage to such conjugated polymers.

An example of a conjugated polymer which can be rendered soluble by themethod according to the invention is a polythiophene. Polythiophenes aresemiconductors, can be easily doped and are quite stable. However, theavailability of these compounds is hampered by solubility problems. Forexample, a polythiophene without any substituents is only soluble up tothe hexamer. Polythiophene of higher molecular weight can be prepared byelectrochemical polymerization but this leads to intractable films aswell as to a polythiophene having a backbone which has many topologicaldefects. Using the method according to the invention an unsubstitutedpolythiophene can be prepared which is substantially free of topologicaldefects and of relatively high molecular weight. A suitable precursorcompound uses tetrahydrothiophene precursor units having arylthio oralkylthio groups as the thermally separable substituents. A ratio of oneprecursor unit to two thiophene units is already sufficient to renderthe precursor compound soluble in many common organic solvents thusallowing polythiophene films of high quality to be obtained on a varietyof substrates.

The polythiophene samples prepared using the method according to theinvention are of high molecular weight. Gel permeation chromatographicanalysis indicates that a number-average molecular weight of at least3500 can be routinely obtained. The optical absorption spectrum shows abroad peak between 350 and 750 nm which is characteristic ofpolythiophene. However, unlike polythiophene samples which are notaccording to the invention, the broad peak shows various shoulders. Thisis considered to be due to the absence of topological defects.

The invention further relates to precursor compounds which can besuitably used in a method according to the invention.

A suitable precursor compound is a soluble precursor compound comprisinga precursor unit consisting of a tetrahydrothiophene or tetrahydrofuranunit having thermally separable substituents --SR₁ and --SR₂, wherein R₁and R₂ are independently selected as an alkyl or aryl group.

Examples of such suitable precursor compounds have already beendescribed hereinabove. Also suitable are of course intermediateprecursor compounds comprising as a precursor unit a dihydrothiophene ordihydrofuran unit which can be obtained from the precursor compoundsdescribed hereinabove by heating such that some, but not all, of thethermally separable substituents are eliminated from the precursorcompounds thus forming the intermediate precursor compounds.

The intermediate compounds need not be obtained by partial thermalconversion of the tetrahydro heterocycle. For example, treatment of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene with2-thienyllithium at -20° C. results in the corresponding dihydrocompound, 3-phenylthio-2,5-di(2-thienyl)-2,3-dihydrohydrothiophene. Thedihydro compounds have lower conversion temperatures than thecorresponding tetrahydro compounds.

The invention also relates to a semiconductor device having asemiconducting layer comprising a polythiophene which is obtainable byusing a method according to the invention.

In a preferred embodiment, the semiconductor device is a field-effecttransistor in which the semiconducting field-effect active layercomprises unsubstituted polythiophene, where the term polythiophene isunderstood to include oligothiophene. By determining current voltagecharacteristics at various gate voltages a pronounced field-effect isobserved. A typical value of the field-effect charge-carrier mobility isapproximately 10⁻⁶ cm² /(Vs) at a bulk conductivity of 8×10⁻⁹ S/cm.These values are typical of amorphous semiconducting polymers processedfrom solution. A good field-effect transistor combines a high mobilitywith a low bulk conductivity.

The invention will be further elucidated with the aid of the followingexamples.

In the drawings:

FIG. 1 shows the absorbance A (in arbitrary units au) as a function ofwavelength λ (in nm) of a conjugated compound (curve A) prepared by themethod according to the invention using a precursor compound (curve B)according to the invention, and

FIG. 2 shows the drain current I_(d) (in A) as a function of the gatevoltage V_(g) (in V) at a drain voltage of -20 V (curve A) and -2 V(curve B) of a field-effect transistor according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Synthesis of 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrofuran##STR2##

2-phenylthioacetic acid

Thiophenol (521 g, 4.736 mol) in 800 ml THF is added to a cold mixtureof sodium hydroxide (400 g, 10 mol) and 2 kg ice. Sodium bromide (50 g)and triethylbenzylammonium chloride (8 g) are added to the 20° C.mixture which is stirred mechanically in a large beaker. Chloroaceticacid (500 g, 5.29 mol) is added portionwise at a temperature of 30-40°C. and at the same time ice is being added in order to moderate theexothermal reaction. At the end of the chloroacetic acid additionanother 80 g sodium hydroxide is added make the mixture basic. The thickpaste is diluted with so much water that a stirrable paste is obtained(the total volume being about 7 l). After stirring for 2 hours andstanding overnight, the paste is acidified with concentratedhydrochloric acid. Air is bubbled through the mixture in order to removethe THF and allow the product to crystallize. It is collected byfiltration and washed with water. After air-drying there is obtained 762g (4.536 mol, 96 %) of 2-phenylthioacetic acid.

2-phenylthioacetyl chloride

A quantity of 762 g (4.536 mol) of 2-phenylthioacetic acid is stirred atroom temperature for 3 hours with thionyl chloride (375 ml, 5.14 mol),then stirred at 40° C. for 3 hours. Rotary evaporation followed bybulb-to-bulb distillation gives 777 g (4.166 mol, 92%) of2-phenylthioacetyl chloride.

2-phenylthio-1-(2-thienyl)-ethanone

To a cooled mixture of 2-phenylthioacetyl chloride (593 g, 3.18 mol),thiophene (286 g, 3.40 mol), and 1500 ml toluene, tin(IV)chloride (875g, 3.36 mol) is added over a period of 2 to 3 hours at temperaturesbetween 0 and 7° C. The mixture is then stirred for 3 hours whereby thetemperature of the reaction mixture rises gradually to room temperature.Some ice is added carefully to the mixture followed by 100 mlconcentrated hydrochloric acid in 1500 ml water. The layers areseparated and the organic layer is washed with 2×500 ml water. Theaqueous layers are extracted with 1 l toluene. The organic layers aredried and rotary evaporated. The residue is stirred with 1500 mlmethanol, then filtered, the solid being washed with 1 l methanol. Aquantity of 421 g of 2-phenylthio-1-(2-thienyl)-ethanone is obtained.The filtrate is rotary evaporated and the residue is purified bybulb-to-bulb distillation. The product, which distils at about 140° C.and 1 mm Hg, is stirred with methanol to give a pure product. Thefiltrate of this crystallization is rotary evaporated and the residue isallowed to stand with some seed crystals. The supernatant liquid ispoured off and the residue is stirred with methanol to give anadditional amount of product. The total yield is 627 g (2.68 mol, 84%).

¹ H NMR (CCl₄): δ4.0 (s, 2H), 6.9-7.6 (m, 8H).

2,3-bis(phenylthio)-1,4-di(2-thienyl)-1,4-butanedione

2-phenylthio-1-(2-thienyl)-ethanone (231.4 g, 0.989 mol) is added at -3to 3° C. in portions over a period of 1.5 hours to a mixture of sodiumhydride (37.5 g, 55-65% dispersion in oil, 1.016 mol maximum) and 950 mlTHF. After stirring for 30 minutes at -5° C., the solution is cooled andcupric chloride (120 g, 0.893 mol) is added at -72° C. The mixture isallowed to warm up slowly with mechanical stirring (after 3 hours theinternal temperature is -10° C. and after 5 hours 5° C.). It issubsequently stirred for 4 hours at 10 to 20° C. before it is heated at43° C. for 11 hours. After rotary evaporation (the THF can be reused)500 ml 2 N hydrochloric acid and 1500 ml toluene are added to theresidue. The mixture is stirred, then filtered under vacuum and thelayers are separated. The organic layer is washed with 2×150 ml water,then dried and evaporated to leave about 120 g residue. The solid isstirred at 50° C. with 1500 ml toluene and the first aqueous layer.Filtration, separation, washing with the other water layers, drying andevaporation gives another 30 g. These combined product fractions containsome starting material. The solid is boiled with 750 ml chloroform, themixture is filtered while hot over a big plug of cotton wool and thefiltrate is rotary evaporated. The undissolved material is boiled oncemore with 500 ml chloroform, the mixture is filtered while hot over aplug of cotton wool and the filtrate is rotary evaporated. To thecombined filtrates, which contain both the meso and dl isomer of2,3-bis(phenylthio)-1,4-di(2-thienyl)-1,4-butanedione, 600 ml tolueneand 3.6 g diethylamine are added and the mixture is stirred for 20 hoursat room temperature. Filtration and washing with toluene gives 137 g ofthe pure meso isomer(0.294 mol, 59%). The filtrate is rotary evaporatedand the residue is purified by bulb-to-bulb distillation. The distillatefraction, which boils at 140° C. and 1 mm Hg is stirred with methanol.This gives 24 g of the starting compound.

¹ H NMR of the dl-isomer (CDCl₃): δ4.65 (s, 2H), 7.0-7.6 (m, 16H).

¹ H NMR of the meso-isomer (CDCl₃): δ4.85 (s, 2H), 7.1-7.8 (m, 16H).

2,3-bis(phenylthio)-1,4-di(2-thienyl)-1,4-butanediol

A mixture of lithium aluminum hydride (30.8 g, 0.789 mol) and 1 l THF iscooled with liquid nitrogen to about -10° C. Meso2,3-bis(phenylthio)-1,4-di-(2-thienyl)-1,4-butanedione (288.88 g, 0.620mol) is added in portions over a period of 30 minutes at temperaturesbetween 0 and -10° C. After the addition the mixture is stirredmechanically for 20 minutes at the same temperature. Ethylacetate (200ml) is added dropwise while cooling well at temperatures below 7° C.,followed by 200 ml acetic acid at the same temperatures. The mixture isstirred for 5 minutes, then poured in 800 ml 4 N hydrochloric acid. Thelayers are separated and the organic layer is washed with 2×250 mlbrine, then dried and rotary evaporated. The aqueous layers areextracted with 1 l ethylacetate, which is dried and combined with thepartially evaporated THF-layer. Rotary evaporation is continued untilabout 500 g residue is left. About 300 ml methanol is added to thissuspension, stirring, filtration and washing with methanol gives 248.72g of 2,3-bis(phenylthio)-1,4-di(2-thienyl)-1,4-butanediol. The filtrateis rotary evaporated and the residue is stirred with some methanol togive another 8.50 g of the same compound, resulting in a total yield of257.22 g (0.547 mol, 88%).

¹ H NMR (CDCl₃): δ2.7 (bs, 2H, exchange with D₂ O), 3.6 (bs, 2H), 5.7(bs, 2H), 6.8-7.2 (m, 16H).

3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrofuran ##STR3##

To a suspension of 2,3-bis(phenylthio)-1,4-di(2-thienyl)-1,4-butanediol(54.0 g, 114.9 mmol) in 350 ml dioxane there is added 8.70 g conc.sulfuric acid in 4 almost equal portions with a 45 minutes intervalbetween each addition. The rather thick suspension is stirredmechanically for 4 days at room temperature whereby a clear solution isobtained. The solution is poured into 500 ml water and the product isextracted with 2×300 ml toluene. The organic layer is washed with 2×250ml water, then dried and rotary evaporated. A quantity of 50 ml ether,followed by 200 ml methanol as well as some seed crystals are stirredinto the residue. The crystallized product is filtered off and washedwith methanol. It weighs 21.83 g. The filtrate is rotary evaporated andthe residue is chromatographed on an aluminum oxide column (16×3 cm)using a 1/1 mixture of toluene and hexane as the eluent, thus obtaininga fraction with an additional amount of the product. This fraction isrotary evaporated and the residue is dissolved in some ether. Methanolis added while stirring, whereupon 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrofuran crystallizes. It isisolated in the usual way and weighs 7.04 g. The total yield is 28.87 g(63.9 mmol, 56%).

¹ H NMR (CDCl₃): δ4.2-4.4 (m, 2H), 5.5 (d, 1H), 5.9 (d, 1H), 6.9-7.4 (m,16H)

The product, 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrofuran, is aprecursor compound having a precursor according to the formula (I)wherein X=O, R₁ =R₂ =Ph, R₃ =R₄ =R₅ =R₆ =H, which can be suitably usedin a method according to the invention. Heating the compound toapproximately 250° C. for 30 minutes yields the conjugated compound2,5-di(2-thienyl)-furan.

EXAMPLE 2

Synthesis of 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene##STR4##

Lawesson's reagent (151.6 g, 0.375 mol) is stirred with 1 l pyridine at50° C. for 1 hours, resulting in a solution, and2,3-bis(phenylthio)-1,4-di(2-thienyl)-1,4-butanediol as obtained inExample 1 (129.52 g, 0.276 mol) is added and the mixture is stirred for1 hours at 50-60° C., resulting in a rather thick suspension. Thesuspension is warmed up to 90° C. over a period of 2 hours and then keptat 90±3° C. for 3 days (becoming a clear solution after 2 days). Thesolution is rotary evaporated (the solvent is used for similarreactions) and the residue is stirred for 30 min with 1 l toluene and750 ml 2 N sodium hydroxide solution. The layers are separated and theorganic layer is washed with 2×300 ml water. The aqueous layers areextracted with 500 ml toluene. The organic layers are dried and rotaryevaporated. The residue is stirred with hexane to which so much tolueneis added that the oil is converted into a crystalline solid. Filtrationand washing gives the crude unsymmetrical isomer which is combined with15.0 g of a similar crude product and then purified over a shortaluminum oxide column using hexane/toluene (1/1) as the eluent. Theeluent is rotary evaporated and the residue is recrystallized fromtoluene/hexane to yield 55.46 g of the unsymmetrical isomer of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene. The filtratesfrom both crystallizations are combined and filtered over an aluminumoxide column (together with some other filtrates from two similarreactions). The elution is carried out using hexane containingincreasing amounts of toluene. Several fractions are obtained. The firstfractions contain an impurity, which probably results from dehydrationof the diol. The next fractions with mainly the symmetrical isomer arecombined, rotary evaporated and the residue is recrystallized fromhexane/toluene. The next column fractions are a mixture of thesymmetrical and unsymmetrical isomer. The next column fractions, whichare enriched in the unsymmetrical isomer, are combined and rotaryevaporated, and the residue is recrystallized from hexane/toluene togive the unsymmetrical isomer. The filtrates of the crystallizations andthe mixed fractions are combined and again purified over an aluminumoxide column. Further purification then gives an additional amount ofthe symmetrical and unsymmetrical isomer. A total amount of 108.01 g ofthe symmetrical and unsymmetrical isomer is thus obtained from 3reactions starting from a total of 248.02 g of2,3-bis(phenylthio)-1,4-di(2-thienyl)1,4-butanediol (0.231 mol, 44%).

₁ H NMR (CDCl₃) of the unsymmetrical isomer: δ4.0-4.2 (m, 2H), 5.1 (d,J=10 Hz, 1H), 5.55 (d, J=4 Hz, 1H), 6.8-7.3 (m, 16H).

¹ H NMR (CDCl₃) of the symmetrical isomer: δ4.2 (d, J=5 Hz, 2H), 5.0 (d,J=5 Hz, 2H), 6.8-7.4 (m, 16H).

The product, 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene,is a precursor compound having a precursor unit according to the formula(I) wherein X=S, R₁ =R₂ =Ph, R₃ =R₄ =R₅ =R₆ =H, which can be suitablyused in a method according to the invention. When heated at 175° C. for30 min, thiophenol is quantitatively eliminated and the conjugatedcompound terthiophene is obtained.

EXAMPLE 3

Synthesis of3,4-bis((4-chloro)phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene##STR5##

Starting from 4-chlorothiophenol, the synthesis of this compound isanalogous to that of the tetrahydrothiophene synthesized in Example 2.

¹ H NMR (CDCl₃): δ3.9-4.1 (m, 2H), 5.05 (d, J=10 Hz, 1H), 5.55 (d, J=4Hz, 1H, 6.8-7.3 (m, 14H).

The product,3,4-bis[(4-chloro)phenylthio]-2,5-di(2-thienyl)-tetrahydrothiophene, isa precursor compound having a precursor unit according to the formula(I), wherein X=S, R₁ =R₂ =4-chlorophenyl, R₃ =R₄ =R₅ =R₆ =H, which canbe suitably used in a method according to the invention. When heated at175° C. for 30 minutes, 4-chlorothiophenol is quantitatively eliminatedand the conjugated compound terthiophene is obtained.

EXAMPLE 4

Synthesis of3,4-bis((4-methyl)phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene##STR6##

Starting from thiocresol, the synthesis of this compound is analogous tothat of the tetrahydrothiophene synthesized in Example 2.

¹ H NMR (CDCl₃): δ2.25 (ss, 6H), 3.9-4.1 (m, 2H), 5.05 (d, J=10 Hz, 1H),5.5.(d, J=4 Hz, 1H), 6.9-7.3 (m, 14H).

The product, 3,4-bis(4-methyl)phenylthio-2,5-di(2-thienyl)-tetrahydrothiophene, is a precursor compound having a precursor unitaccording to the formula (I), wherein X=S, R₁ =R₂ =4-methylphenyl, R₃=R₄ =R₅ =R₆ =H, which can be suitably used in a method according to theinvention. When heated at 175° C. for 30 minutes, 4-methylthiophenol iseliminated and the conjugated compound terthiophene is obtained.

Further precursor compounds which have been prepared in an analogousmanner are 3,4-diethyl-2,5-di(2-thienyl)-tetrahydrothiophene and3,4-di(tert-butyl)-2,5-di(2-thienyl)-tetrahydrothiophene. Thermalconversion proceeds under substantially identical conditions asmentioned above.

Further precursor compounds which can be prepared analogously are3,4-bis ((4-nitro)phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene and3,4-bis(4-pyridyl)-2,5-di(2-thienyl)-tetrahydrothiophene.

EXAMPLE 5

Synthesis of the dimer of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene ##STR7##

3,4-bis(phenylthio)-2,5-di(5-bromo-2-thienyl)-tetrahydrothiophene

To an ice-cooled solution of the unsymmetrical isomer of 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene as obtained inExample 2 (20.0 g, 42.8 mmol) in 80 ml DMF there is added in portions inabout 5 minutes 20.5 g (115.2 mmol) N-bromosuccinimide (NBS). After partof the NBS had been added the temperature rose to 11° C. The addition ofthe remainder of the NBS was performed at 5-7° C. The mixture wasstirred for 30 minutes, allowing the temperature to rise to 12° C. Thesolution is cooled and 4.0 g sodium dithionite is added. The mixture isstirred for 30 minutes at 5° C., then 30 minutes at 5 to 15° C. Themixture is cooled with ice and 50 ml water is added, followed by 50 mltoluene. After stirring for 5 minutes the mixture is poured in 300 mlwater and 300 ml toluene. The layers are separated, the aqueous layer isextracted with 300 ml toluene, and the organic layers are washed with2×100 ml water, then dried and rotary evaporated. The residue isfiltered over a short aluminum oxide column using hexane/toluene (1/1)as the eluent. Rotary evaporation of the eluate, followed bycrystallization of the residue from hexane/toluene affords 17.17 g ofthe unsymmetrical isomer of3,4-bis(phenylthio)-2,5-di(5-bromo-2-thienyl)-tetrahydrothiophene (27.4mmol, 64%).

¹ H NMR (CDCl₃): δ3.9-4.2 (m,2H), 4.95 (d, J=10 Hz, 1H), 5.4 (d, J=4 Hz,1H), 6.8-6.9 (m, 4H), 7.0-7.3 (m, 10H).

3,4-bis(phenylthio)-2(5-bromo-2-thienyl)-5(5-tributylstannyl-2-thienyl)-tetrahydrothiophene

A solution of3,4-bis(phenylthio)-2,5-di(5-bromo-2-thienyl)-tethrahydrothiophene (6.26g, 10.0 mmol) in 80 ml THF is cooled to at least -70° C. n-Butyllithium(5.0 ml, 12.5 mmol) in hexane is added in 2 minutes at thesetemperatures. The mixture is then stirred for 5 minutes at -80° C.,after which tributyltin chloride (90%, 4.78 g, 14.7 mmol) in 5 ml THF isadded over a 1 minutes period at -70° C. or less. The mixture is stirredfor 30 minutes at -60 to -80° C., then allowed to warm up to -10° C.Water and hexane are added and the mixture is worked up in the usual wayby pouring the mixture in water, extracting with hexane, washing withwater, drying and rotary evaporating. The crude product ischromatographed over an aluminum oxide column (25×2 cm), using hexanecontaining increasing amounts of toluene as the eluent. Hexane elutessome impurities and the bistin compound, hexane containing some tolueneelutes the product, hexane/toluene (1/1) elutes some starting dibromide.The monotin compound weighs 6.05 g (7.24 mmol, 72%), it still containssome minor impurities.

dimer of 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene##STR8##

A quantity of 6.05 g (7.24 mmol) of3,4-bis(phenylthio)-2(5-bromo-2-thienyl-5(5-tributylstannyl-2-thienyl)-tetrahydrothiophene,5.66 g of3,4-bis(phenylthio)-2,5-di(5-bromo-2-thienyl)-tetrahydrothiophene (9.04mmol), dichlorobistriphenylphosphinepalladium (350 mg, 0.50 mmol) and 40ml N,N-dimethylacetamide are heated for 48 hours at 65-70° C. Zincpowder (4.0 g) is added and the mixture is stirred for another 24 hoursat 65-70° C. The solvent is removed under vacuum at 60° C., chloroformis added to the residue and the mixture is filtered over a shortaluminum oxide column. The filtrate is rotary evaporated and the residueis chromatographed over an aluminum oxide column (20×3 cm) using hexanecontaining increasing amounts of toluene as the eluent. This givesseveral fractions which are rotary evaporated. The residue is dissolvedin a small amount of toluene and this solution is then added, whilestirring, to an excess of methanol. This causes precipitation of theproducts, which are subsequently analyzed by HPLC (Nucleosil 5NO₂column, hexane/dichloromethane, 20/80 as the eluent, flow 0.5 ml/min).The monomer 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophenehas a retention time of 3.32 minutes, the dimer thereof a retention timeof 5.29 min and the trimer thereof a retention time of 4.90 minutes. Twomajor fractions were obtained, 1.00 g dimer of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene (almost pure)and 4.37 g dimer (containing 5-10% of the trimer).

The dimer and trimer of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene are bothprecursor compounds having precursor units according to the formula (I),wherein X=S, R₁ =R₂ =phenyl, R₃ =R₄ =R₅ =R₆ =H, and showing excellentsolubility in chloroform, dichloromethane, THF and the like. They can besuitably used in a method according to the invention. When heated at200-250° C. for 15 minutes, thiophenol is quantitatively eliminated anda thiophene-containing conjugated compound, sexithiophene, is formedfrom the dimer, whereas from the trimer nonithiophene is formed.

EXAMPLE 6

Synthesis of the trimer of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene ##STR9##

3,4-bis(phenylthio)-2,5-di(5-tributylstannyl-2-thienyl)-tetrahydrothiophene

A solution of3,4-bis(phenylthio)-2,5-di(5-bromo-2-thienyl)-tetrahydrothiophene (7.96g, 12.71 mmol) as obtained in Example 5, in 100 ml THF is cooled to atemperature below -80° C. n-Butyllithium (10.7 ml, 26.75 mmol) in hexaneis added in 2 minutes at these temperatures. The mixture is then stirredfor 5 minutes at -80° C. (longer stirring times lead to side-reactions,probably resulting from deprotonation at the 2-position of thetetrahydrothiophene ring), after which tributyltin chloride (90%, 10.7g, 32.9 mmol) in 10 ml THF is added over a 3 minute period at atemperature below -70° C. The mixture is stirred for 30 minutes at -60to -80° C., then allowed to warm up to -25° C. Water and hexane areadded and the mixture is worked up in the usual way. The crude productis chromatographed over an aluminum oxide column (30×4 cm), using hexanecontaining increasing amounts of toluene as the eluent. Hexane elutessome impurities, hexane containing a trace of toluene elutes the bistincompound,3,4-bis(phenylthio)-2,5-di(5-tributylstannyl-2-thienyl)-tetrahydrothiophene,which still contains some minor impurities. The yield is 7.13 g (6.82mmol, 54%).

trimer of 3,4bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene##STR10##

The bistin compound (3.73 g, 3.57 mmol) obtained above,3,4-bis(phenylthio)-2,5-di(5-bromo-2-thienyl)-tetrahydrothiophene (4.52g, 7.22 mmol), the catalyst dichlorobistriphenylphosphinepalladium (315mg, 0.45 mmol) and 25 ml N,N-dimethylacetamide are mixed and heated for20 hours at 80° C. Another 100 mg catalyst (0.14 mmol) is added and themixture is heated for 24 hours at 85° C. Zinc powder (2.0 g) is addedand the mixture is stirred for 24 hours at 85° C. (Note: a lowertemperature is beneficial, because some elimination of thiophenol isobserved at 85° C.). The solvent is removed under vacuum, chloroform isadded to the residue and the mixture is filtered over a short aluminumoxide column. The filtrate is rotary evaporated and the residue ischromatographed over an aluminum oxide column using hexane containingincreasing amounts of toluene as the eluent. This gives severalfractions which are rotary evaporated. The residue is dissolved in asmall amount of toluene and this solution is then added, while stirring,to an excess of methanol. This causes precipitation of the products,which are subsequently analyzed by HPLC. The fractions consist of thedimer, dimer/trimer mixtures and almost pure trimer of 3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene.

EXAMPLE 7

The synthesis of3,4-bis(phenylthio)-2,5-di[5-(2,2'-bithienyl)]-tetrahydrothiophene##STR11##

A 50 ml flask, equipped with condenser, magnetic stirrer and nitrogeninlet was charged with 56.2 mg (0.08 mmol) ofdi(triphenylphosphine)palladium(II)dichloride in 20 ml dry THF, followedby 500 mg (0.8 mmol) of the dibromo compound as obtained in Example 5 in5 ml THF. Slowly, 670 mg (1.83 mmol) of 2-tributylstannylthiophene in 5ml of THF was added under nitrogen. The synthesis of2-tributylstannylthiophene is described by Kotani et al. in J.Organomet. Chem., 1992, 429, p403. The reaction mixture was stirred for16 hours at 60° C. After cooling to room temperature, the THF wasremoved in vacuo, followed by the addition of 40 ml water with a fewdrops of HCl. The mixture was extracted three times with 20 ml ether.The combined organic phases were washed with water and saturated NaClsolution. The organic layer was dried with magnesium sulfate and removedin vacuo. The product,3,4-bis(phenylthio)-2,5-di[5-(2,2'-bithienyl)]-tetrahydrothiophene, waspurified by column chromatography using pentane:ethyl acetate [9: 1] aseluent. Yield 203 mg (39.8 %).

¹ H NMR: δ=6.9-7.7 (m, 20H), 6.0 (d,1H), 5.1 (d, a1H), 4.5 (q, 2H) ppm.

The product,3,4-bis(phenylthio)-2,5-di[5-(2,2'-bithienyl)]-tetrahydrothiophene, is aprecursor compound having a precursor unit according to the formula (I),wherein X=S, R₁ =R₂ =phenyl, R₃ =R₄ =R₅ =R₆ =H, which can be suitablyused in a method according to the invention. Heating at 200-250° C. for15 min results quantitatively in quinquethiophene.

EXAMPLE 8

The synthesis of 3,4-bis(phenylthio)-2,5-di[5-(2,2':5',2"-terthienyl)]-tetrahydrothiophene ##STR12##

The synthesis of this tetrahydrothiophene is analogous to the synthesisdescribed in Example 7, with this difference that2-tributylstannylthiophene is replaced by2-thienyl-5-tributylstannylthiophene, the synthesis of which isdescribed by Hark et al. in Tetrahedron Lett., 1994, 35, p7719. Theyield is 28.1 %.

¹ H NMR: δ=6.9-7.6 (m, 24 H), 6.0 (d, 1H), 5.1 (d, 1H),4.5 (q, 2H) ppm.

The product,3,4-bis(phenylthio)-2,5-di[5-(2,2':5',2"-terthienyl)]-tetrahydrothiophene,is a precursor compound having a precursor unit according to the formula(I), wherein X=S, R₁ =R₂ =phenyl, R₃ =R₄ =R₅ =R₆ =H, which can besuitably used in a method according to the invention. Heating at200-250° C. for 15 minutes results quantitatively in septithiophene.

EXAMPLE 9

The synthesis of the tetrahydrothiophene according to the formula:##STR13##

The synthesis of this tetrahydrothiophene is analogous to the synthesisdescribed in Example 7, with this difference that2-tributylstannylthiophene is replaced by2-(5-tributylstannyl-2-thienyl)-5-(2-thienyl)thiophene, which can beprepared in an analogous manner. The yield is 117 mg (86.3%).

¹ H NMR: δ=6.9-7.7 (m, 28 H), 6.0 (d, 1H), 5.1 (d, 1H), 4.5 (q, 2H) ppm.

The product is a precursor compound having a precursor unit according tothe formula (I), wherein X=S, R₁ =R₂ =phenyl, R₃ =R₄ =R₅ =R₆ =H, whichcan be suitably used in a method according to the invention. Heating at200-250° C. for 15 minutes results quantitatively in nonithiophene.

EXAMPLE 10

Synthesis ofpoly[3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene]##STR14##

The dibromide3,4-bis(phenylthio)-2,5-di(5-bromo-2-thienyl)-tetrahydrothiophene (0.5g, 0.8 mmol), dissolved in 5 ml dimethylformamide, was added to asolution of 2,2'-bipyridyl (315 mg; 2.0 mmol) and Ni(cyclooctadiene)₂(550 mg, 2.0 mmol) in 20 ml of dimethylformamide. The mixture wasstirred for 24 hours at 60° C. under an inert atmosphere andprecipitated into methanol (200 ml). The solid material was dissolvedagain in chloroform, washed with water, reprecipitated in ether anddried in vacuum overnight. Yield: 245 mg (65.7%).

¹ H NMR (DMSO) of the unsymmetrical polymer: δ4.3 (2H), 4.9 (1H), 5.85(1H), 6.8-7.2 (16H).

The molecular weight of the precursor compound thus obtained isdetermined by gel permeation chromatography (GPC) as follows:

A solution of the polymer in chloroform (2 mg/ml) kept at a temperatureof 40° C. is filtered over a 0.5 micron Millex filter and analysed on aGPC system, comprising a PL gel 5 mm Guard column connected in series toa second PL gel 5 mm Mixed C column and a UV/VIS detector set at 254 nm.The calibration is performed using polystyrene standards (Easical). Thepolymer elutes between 14 and 18 minutes with a maximum at 16 minutes.From the chromatograms, the number-average molecular weight of thepolymer is calculated to be 3500, the weight-average molecular weightamounts to 6400 and the dispersion amounts to 1.8. By comparison, analmost pure sample of the trimer, which corresponds to the polymer withn=1, has a retention time of 19 minutes. From these data, thenumber-average molecular weight of the trimer is calculated to be 220,the weight-average molecular weight amounts to 230 and the dispersionamounts to 1.1. Theoretically, these numbers should equal 460, 460, and1 respectively, which demonstrates that, as a person skilled in the artwill expect, the GPC analysis of the polymer underestimates themolecular weight. In fact, the molecular weight appears to beunderestimated by a factor of 2.

The product,poly[3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene], is aprecursor compound having precursor units according to the formula (I),wherein X=S, R₁ =R₂ =phenyl, R₃ =R₄ =R₅ =R₆ =H, which can be suitablyused in a method according to the invention. It has excellent solubilityin common organic solvents like chloroform, THF and dichloromethane. Athermogravimetric analysis (Perkin Elmer 7 Series Thermal AnalysisSystem), whereby 1.538 mg of the polymer is heated at a rate 10° C. permin from 30 to 500° C., shows that phenylthiol is eliminated betweenapproximately 190 and 270° C. thus forming the conjugated compoundpolythiophene.

FIG. 1 shows the absorbance A (in au) as a function of the wavelength λ(in nm) of a conjugated compound (curve A) prepared by the methodaccording to the invention using a precursor compound (curve B)according to the invention. The spectra are recorded by means of aPerkin Elmer Lambda 9 spectrophotometer. The spectrum represented bycurve B corresponds to the precursor compoundpoly[3,4-bis(phenylthio)-2,5-di (2-thienyl)-tetrahydrothiophene]. Afterevacuating for 2 hours at 250° C. and 10⁻⁵ Torr, the spectrum in curve Ais obtained which shows a broad peak between 350 and 750 nm which ischaracteristic of polythiophene. However, unlike polythiophene sampleswhich are not according to the invention, the broad peak shows variousshoulders. This is considered to be due to the absence of topologicaldefects.

EXAMPLE 11

Using the polymer obtained in Example 10, ametal-insulator-semiconductor field-effect transistor (MISFET) ismanufactured as follows:

A highly doped n⁺⁺ -type silicon wafer which acts as the gate contact,is provided with a 200 nm insulating layer of thermally grownsilicondioxide. Using standard lithographic techniques, a set ofinterdigitated source and drain gold contacts is provided on top of theinsulator to give channel widths in the range of 3-20 mm and channellengths in the range of 2-20 microns, thus rendering the MISFETsubstrate complete. Source to drain resistances are in excess of 10¹²Ohms. Subsequently, a 1 wt % solution of poly[3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene] as prepared inExample 10 in chloroform is spincoated onto the MISFET substrate (3 s at300 rpm followed by 30 seconds at 1000 rpm) to give a 50-150 nm thickprecursor layer. The precise thickness is determined by means of aquartz substrate onto which the polymer is spincoated in an identicalmanner. The semiconducting layer is formed from the precursor layer byheating at a temperature of 250° C. at 10⁻⁵ Torr for 2 hours. TheMISFET--in fact a set of MISFETS is formed--is now complete. A currentvoltage characteristic of a MISFET whose channel width is 10 mm andchannel length is 5 microns is then determined by connecting a currentmeasuring device and both the gate and drain to a voltage source.Starting at a gate bias V_(g) of +20 V, the gate bias is swept to -20 Vand back to +20 V. This is done at different drain voltages, the sourcebeing grounded at all times. In order to avoid doping of thesemiconducting layer, samples are evacuated prior to measurement and themeasurement itself is performed under nitrogen or vacuum. The result isshown in FIG. 2.

FIG. 2 shows the drain current I_(d) (in A) as a function of the gatevoltage V_(g) (in V) at a drain voltage of 20 V (curve A) and -2 V(curve B) of a field-effect transistor according to the invention.

Using standard equations which model the current voltage characteristicof the MISFET taken at a drain voltage of -2 V, a voltage at which theMISFET is operating in the linear regime, the conductivity is calculatedto be 8×10⁻⁹ S/cm and the charge-carrier mobility is calculated to be10⁻⁶ cm² /(Vs).

EXAMPLE 12

Example 11 is repeated with this difference that the polymerpoly[3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene] is(partially) replaced by either the dimer of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene, which afterconversion yields sexithiophene, the trimer of3,4-bis(phenylthio)-2,5-di(2-thienyl)-tetrahydrothiophene, which afterthermal conversion yields nonithiophene, or mixtures thereof. Sometypical values of the conductivity ρ and charge-carrier mobility μ arecollected in Table 1.

                  TABLE 1                                                         ______________________________________                                        polymer (wt %)                                                                         dimer (wt %)                                                                            trimer (wt %)                                                                            ρ (S/cm)                                                                         μ (cm.sup.2 /(Vs)                     ______________________________________                                        0        100       0            2 × 10.sup.-8                                                                  1 × 10.sup.-5                      0 0 100   5 × 10.sup.-7   6 × 10.sup.-5                           50 50 0   6 × 10.sup.-6   5 × 10.sup.-4                           25 0 75 9.5 × 10.sup.-7 2.5 × 10.sup.-4                         ______________________________________                                    

From the results in this table it is apparent that the most favourableelectrical properties are obtained, a low conductivity in combinationwith a high mobility being particularly desirable, if mixtures ofpolymers and oligomers of thiophene are used.

We claim:
 1. A method of preparing a thiophene-containing orfuran-containing conjugated compound, whereby a soluble precursorcompound, comprising a precursor unit having thermally separablesubstituents, is heated such that said thermally separable substituentsare eliminated from said precursor compound while converting saidprecursor compound into said thiophene-containing or furan-containingconjugated compound, wherein in that the precursor unit used is atetrahydrothiophene or tetrahydrofuran unit having thermally separablesubstituents --SR₁ and --SR₂, wherein R₁ and R₂ are independentlyselected as an alkyl or aryl group.
 2. A method as claimed in claim 1,wherein in that, before the precursor compound is heated, a solutioncomprising the precursor compound is prepared and provided onto asubstrate.
 3. A method as claimed in claim 1, wherein in that R₁ and R₂are independently selected from the group consisting of phenyl,4-methylphenyl, 4-chlorophenyl, 4-nitrophenyl, 4-pyridyl, ethyl andtert-butyl.
 4. A method as claimed in claim 1, wherein in that R₁ ischosen equal to R₂.
 5. A method as claimed in claim 1, wherein theprecursor unit is a 2,5-dithienyltetrahydrothiophene or2,5-dithienyltetrahydrofuran unit.
 6. A method as claimed in claim 1,wherein in that the thiophene-containing or furan-containing conjugatedcompound being prepared is a polymer having a substantially conjugatedbackbone.
 7. A method as claimed in claim 6, wherein in that the polymerbeing prepared is a polythiophene.